There exist coordinate input apparatuses which are used to input coordinates to a coordinate input surface by using a pointer (e.g., a dedicated input pen or finger) to control a connected computer or write characters and graphics.
Conventionally, as coordinate input apparatuses of this type, touch panels of various schemes have been proposed or become commercially available. These apparatuses are widely used because a terminal such as a personal computer can easily be operated on the screen without using any special tool.
There are various coordinate input schemes using, e.g., a resistive film or an ultrasonic wave. A scheme using light is disclosed in, e.g., U.S. Pat. No. 4,507,557. In U.S. Pat. No. 4,507,557, a retroreflecting sheet is provided outside the coordinate input region. By illumination units which are arranged at the corners of the coordinate input region to emit light and light receiving units which receive light, the angles between the light receiving units and a shielding object such as a finger that shields light in the coordinate input region are detected. On the basis of the detection result, the point position of the shielding object is determined.
Japanese Patent Laid-Open No. 2000-105671 or Japanese Patent Laid-Open No. 2001-142642 discloses a coordinate input apparatus which includes a retroreflecting member arranged on the periphery of the coordinate input region and detects the coordinates of a point (light-shielded portion) where retroreflected light is shielded.
In the apparatus of, e.g., Japanese Patent Laid-Open No. 2000-105671, the peak of the light-shielded portion by the shielding object, which is received by the light receiving unit, is detected by waveform processing such as differentiation. The angle of the light-shielded portion with respect to the light receiving unit is detected, and the coordinates of the shielding object are calculated from the detection result. In Japanese Patent Laid-Open No. 2001-142642, comparison with a specific level pattern is done to detect one end and the other end of a light-shielded part, and the center of the coordinates is detected.
The scheme of calculating coordinates by detecting a light shielding position as in U.S. Pat. No. 4,507,557, Japanese Patent Laid-Open No. 2000-105671 and Japanese Patent Laid-Open No. 2001-142642 will be referred to as a light shielding scheme hereinafter.
Such a coordinate input apparatus of light shielding scheme is required to allow simultaneous operations of a plurality of operators to increase the convenience for efficient use in, e.g., a conference especially when the size of the coordinate input region is large. Hence, coordinate input apparatuses capable of coping with a plurality of simultaneous inputs have been devised.
To simultaneously input a plurality of coordinate points, in Japanese Patent Laid-Open No. 2002-055770, Japanese Patent Laid-Open No. 2003-303046 and Japanese Patent Registration No. 2896183, the angles of a plurality of light-shielded portions are detected by one light receiving sensor. Several input coordinate candidates are calculated on the basis of the combinations of the sensor angles. An actually input coordinate point is determined from the input coordinate candidates.
In, e.g., two-point input, a maximum of four coordinate points are calculated as input coordinate candidates. Of the four points, two actually input coordinate points are determined and output. That is, actual input coordinate points and false input coordinate points are discriminated from the plurality of input coordinate candidates, and final input coordinate points are determined. This determination will be referred to as “truth determination” here.
As a detailed method of truth determination, in Japanese Patent Laid-Open No. 2003-303046 or Japanese Patent Registration No. 2896183, first and second sensors are provided at the two ends of one side of a conventional coordinate input region while being spaced part by a distance enough to accurately calculate coordinates pointed in the coordinate input region. In addition, a third sensor is provided between the first and second sensors while being spaced part from them by a distance enough to accurately calculate coordinates pointed in the input region. On the basis of angle information in the third sensor which is different from those of the first and second sensors, truth is determined for a plurality of pieces of angle information detected by the first and second sensors.
Another method has also been proposed in which a plurality of sensor units are arranged at a predetermined interval around a coordinate input region to observe almost the same region in almost the same direction. Even when a plurality of light shielding shadows overlap, each shadow is never detected while being completely hidden by another shadow. Still another method has also been proposed in which when a plurality of shadows overlap, the direction of each shadow is detected by observing one end of the shadow.
In the above-described coordinate input apparatuses of light shielding scheme, all sensors must accurately detect the number of shadows formed by shielding light and their positions and intensities without inconsistency. However, the shadow intensity (the rate of light intensity decrease by light shielding) does not always take the same value for the sensors because of the positional relationship between the sensors and the retroreflecting member and the positional relationship between the light projecting units and the light receiving units in each sensor. “Light shielding depth”, “light shielding ratio”, and “light shielding rate” will also be used as synonyms to shadow intensity hereinafter (the rate of light intensity decrease by light shielding).
If the light shielding depth of a shadow changes, for example, the shadow which should be detected simultaneously by a plurality of predetermined sensors may be detected at a predetermined position by a specific sensor but may not be detected at the predetermined position by another specific sensor. That is, the shadow is missed.
When the shadow that must be detected cannot be detected by one of the sensors, the input coordinates cannot be detected accurately. The influence of this problem is especially serious when a plurality of inputs are done simultaneously by a plurality of pointers. For example, if a shadow which should be detectable by a given sensor cannot be detected, the sensor erroneously senses that the shadow overlaps another shadow. As a result, coordinates at an impossible position, i.e., a wrong position different from the actual input position of the pointer may be detected.